Extrinsic base doping for bipolar junction transistors

ABSTRACT

Device structure and fabrication methods for a bipolar junction transistor. A base layer is formed and an emitter is formed on a first portion of the base layer. A dopant-containing layer is deposited on a second portion of the base layer. Dopant is transferred from the dopant-containing layer into the second portion of the base layer to define an extrinsic base of the device structure.

BACKGROUND

The invention relates generally to semiconductor devices and integrated circuit fabrication and, in particular, to fabrication methods and device structures for a bipolar junction transistor.

Bipolar junction transistors may be found, among other end uses, in high-frequency and high-power applications. In particular, bipolar junction transistors may find specific end uses in amplifiers for wireless communications systems and mobile devices, switches, and oscillators. Bipolar junction transistors may also be used in high-speed logic circuits. Bipolar junction transistors are three-terminal electronic devices that include an emitter, an intrinsic base, and a collector defined by regions of different semiconductor materials. In the device structure, the intrinsic base situated between the emitter and collector. An NPN bipolar junction transistor may include n-type semiconductor material regions constituting the emitter and collector, and a region of p-type semiconductor material constituting the intrinsic base. A PNP bipolar junction transistor includes p-type semiconductor material regions constituting the emitter and collector, and a region of n-type semiconductor material constituting the intrinsic base. In operation, the base-emitter junction is forward biased and the base-collector junction is reverse biased. The collector-emitter current may be controlled by the base-emitter voltage.

Improved fabrication methods and device structures are needed for a bipolar junction transistor.

SUMMARY

In an embodiment of the invention, a method is provided for fabricating a device structure. A base layer is formed and an emitter is formed on a first portion of the base layer. A dopant-containing layer is deposited on a second portion of the base layer. Dopant is transferred from the dopant-containing layer into the second portion of the base layer to define an extrinsic base of the device structure.

In an embodiment of the invention, a device structure is provided for a bipolar junction transistor. The device structure includes a base layer comprised of a semiconductor material, an emitter on a first portion of the base layer, and a dopant-containing layer on a second portion of the base layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate various embodiments of the invention and, together with a general description of the invention given above and the detailed description of the embodiments given below, serve to explain the embodiments of the invention.

FIGS. 1-5 are cross-sectional views of a portion of a substrate at successive fabrication stages of a processing method for fabricating a device structure in accordance with an embodiment of the invention.

DETAILED DESCRIPTION

With reference to FIG. 1 and in accordance with an embodiment of the invention, a bipolar junction transistor 10 is fabricated using a substrate 11. The substrate 11 comprises a single-crystal semiconductor material usable to form the devices of an integrated circuit. For example, the substrate 11 may be comprised of a wafer of a monocrystalline silicon-containing material, such as a single-crystal silicon wafer, or a device layer of a silicon-on-insulator wafer. The semiconductor material constituting the substrate 11 may comprise an electrically-active dopant that alters its electrical properties and may also include an optional epitaxial layer at its top surface 11 a. The substrate 11 may comprise an n-type dopant from Group V of the Periodic Table (e.g., phosphorus (P), arsenic (As), or antimony (Sb)) in a concentration that is effective to impart n-type conductivity.

A base layer 12 of the bipolar junction transistor 10 is located on the top surface 11 a of substrate 11. The base layer 12 may be comprised of a different semiconductor material than the collector 14 and may have an opposite conductivity type from the collector 14. The base layer 12 may be comprised of a semiconductor material, such as silicon-germanium (SiGe) in an alloy with a content of silicon (Si) ranging from 95 atomic percent to 50 atomic percent and a content of germanium (Ge) ranging from 5 atomic percent to 50 atomic percent. The germanium content of the base layer 12 may be uniform across the thickness of base layer 12 or graded and/or stepped across the thickness of base layer 12. The semiconductor material of the base layer 12 may comprise a dopant, such as a p-type dopant selected from Group III of the Periodic Table (e.g., boron) in a concentration that is effective to impart p-type conductivity and, optionally, carbon (C) to suppress the outdiffusion of the p-type dopant. The base layer 12 may be formed from a semiconductor material layer deposited using a low temperature epitaxial (LTE) growth process, such as vapor phase epitaxy (VPE).

An emitter 16 of the bipolar junction transistor 10 is located on a top surface 12 a of the base layer 12. The emitter 16 may be comprised of a different semiconductor material than the base layer 12 and may have an opposite conductivity type from the base layer 12. For example, the emitter 16 may lack germanium that is present in at least a portion of the base layer 12. In a representative embodiment, the emitter 16 may be comprised of a semiconductor material, such as silicon, deposited by chemical vapor deposition (CVD) and may be doped with a concentration of an n-type dopant selected from Group V of the Periodic Table (e.g., phosphorus (P) or arsenic (As)) in a concentration that is effective to impart n-type conductivity.

Trenches 18, 20 extend to a shallow depth beneath the top surface 11 a of the substrate 11 and may establish boundaries for a collector 14 and collector contacts 22, 24 that are located beneath the top surface 11 a of the substrate 11. The trenches 18, 20 may be formed by wet chemical etching process, a dry etching process (e.g., reactive-ion etching (RIE)), or a combination of these etching processes conducted in one or more steps using one or more etch chemistries. The trenches 18, 20 are subsequently used to form trench isolation regions.

The collector 14 may be comprised of a section of the material of the substrate 11 located between the trenches 18, 20 and may contain a concentration of an n-type dopant in a concentration that is effective to impart n-type conductivity. The dopant concentration of the collector 14 may be elevated to enhance its electrical conductivity by ion implantation or the growth of a doped epitaxial layer at the top surface 11 a of the substrate 11. The collector contacts 22, 24 are separated from the collector 14 by the trenches 18, 20. A sub-collector 26 extends laterally from the collector 14 beneath the trenches 18, 20 to couple the collector 14 with the collector contacts 22, 24. The collector contacts 22, 24 and sub-collector 26 are also comprised of the semiconductor material of the substrate 11 and have the same conductivity type as the semiconductor material comprising the collector 14.

The bipolar junction transistor 10 is characterized by a vertical architecture that includes the emitter 16, the collector 14, and an intrinsic base 28 defined by a central section of the base layer 12 that is positioned between the emitter 16 and collector 14. The intrinsic base 28 is coextensive with the collector 14 along a junction 30, and the emitter 16 is coextensive with the intrinsic base 28 along another junction 32. Sections 34, 36 of the base layer 12, which are peripheral to the intrinsic base 28, may collectively define an extrinsic base 38 that is coupled with the intrinsic base 28. In the representative device structure, the sections 34, 36 of the base layer 12 defining the extrinsic base 38 at least partially overhang the trenches 18, 20 and the trench isolation regions subsequently formed in the trenches 18, 20. The bipolar junction transistor 10 may be characterized as a heterojunction bipolar transistor (HBT) if two or all three of the collector 14, the emitter 16, and the intrinsic base 28 are comprised of semiconductor materials with different bandgaps.

A dielectric layer 40 is located on the exposed surfaces of the base layer 12, the emitter 16, and the substrate 11, and a dielectric layer 42 is located on the dielectric layer 40. The dielectric layers 40, 42 may be comprised of different electrical insulator characterized by a dielectric constant (e.g., a permittivity) characteristic of a dielectric material. In one embodiment, the dielectric layer 40 may be comprised of silicon dioxide (SiO₂) deposited using a rapid thermal process or formed by thermal oxidation, and dielectric layer 42 may be comprised of an electrical insulator, such as silicon nitride (Si₃N₄), that is conformally deposited by chemical vapor deposition. A directional anisotropic etching process, such as RIE, is used to preferentially remove the dielectric layer 42 from horizontal surfaces, such as the top surface 12 a of the base layer 12, and partially horizontal surfaces, such as the inclined surfaces inside the trenches 18, 20. The preferential removal uncovers sections of the dielectric layer 40, such as sections of the dielectric layer 40 on top surface 12 a of the base layer 12 and, in particular, on the sections 34, 36 of the base layer 12 defining the extrinsic base 38. Spacers 42 are formed from the dielectric layers 40, 42 on the vertical sidewalls of the emitter 16 by the preferential removal of the dielectric layer 42.

With reference to FIG. 2 in which like reference numerals refer to like features in FIG. 1 and at a subsequent fabrication stage of the processing method, the sections 34, 36 of the base layer 12 defining the extrinsic base 38 are not covered by the emitter 16 and the spacers 42, and project laterally from the intrinsic base 28 that is covered by the emitter 16 and the spacers 42. These sections 34, 36, which are used to contact the covered intrinsic base 28, may be modified to increase their electrical conductivity without the necessity for an ion implantation process.

To that end, a mask layer 44 may be applied to a top surface of the substrate 11. The mask layer 44 may comprise, for example, a photoresist that is applied with a spin coating process, pre-baked, exposed to a radiation projected through a photomask, baked after exposure, and developed with a chemical developer to define a pattern with openings 46, 48. An etching process may be used to remove the dielectric layer 40 from the top surface 12 a of the base layer 12 at positions consistent with the pattern of openings 46, 48 to define windows 50, 52 in the dielectric layer 40. The etching process may comprise a wet chemical etch or a dry etch, and may rely on a given etch chemistry. The windows 50, 52 in the dielectric layer 40 expose respective surface areas on the top surface 12 a of the base layer 12 and, more specifically, exposed surface areas on the sections 34, 36 of the base layer 12 defining the extrinsic base 38. These exposed surface areas on the sections 34, 36 of the base layer 12 are accessible during subsequent processing stages for increasing the electrical conductivity of the extrinsic base 38.

With reference to FIG. 3 in which like reference numerals refer to like features in FIG. 2 and at a subsequent fabrication stage of the processing method, the mask layer 44 may be removed following the etching process. If comprised of a photoresist, the mask layer 44 may be removed by ashing or solvent stripping, followed by a conventional cleaning process.

A layer 54 is deposited on the top surface 12 a of the base layer 12 at locations coinciding with the exposed surface areas inside of windows 50, 52 in the dielectric layer 40. The dielectric layer 40 functions as a hardmask during the deposition. The layer 54 contains a dopant that can be introduced into the semiconductor material of the sections 34, 36 of the base layer 12 defining the extrinsic base 38 and activated to elevate the electrical conductivity. In an embodiment, the dopant contained in the layer 54 may comprise a p-type dopant selected from Group III of the Periodic Table (e.g., boron (B), gallium (Ga) or both) in a concentration that is effective to impart p-type conductivity concentration. The attributes of the layer 54, such as thickness, may be selected to add a targeted concentration of the dopant to all or part of the sections 34, 36 of the base layer 12. In an embodiment, the dopant-containing layer 54 may have a thickness on the order of 1 nanometer to 10 nanometers when deposited, and the diffused length of the dopant from the dopant-containing layer 54 into the base layer 12 during the subsequent annealing is greater this original thickness.

In a specific embodiment, the layer 54 may be comprised of elemental boron deposited in a solid phase by chemical vapor deposition using diborane (B₂H₆) as the gas source and hydrogen (H₂) or nitrogen (N₂) as the carrier gas. The deposition may be conducted at either atmospheric pressure or at a reduced pressure for deposition temperatures ranging from 400° C. to 800° C. The elemental boron in the layer 54 may be in an amorphous state. Through the selection of the deposition conditions, the boron from the gas source primarily deposits as the layer 54 and negligible diffusion occurs into the semiconductor material of the base layer 12. For embodiments in which the deposition of the layer 54 is selective, boron atoms do not adsorb as a distinct layer on the material constituting the dielectric layer 40. However, layer 54 deposits on the dielectric-free surface areas of the top surface 12 a inside the windows 50, 52.

In an alternative embodiment, the layer 54 may be comprised of elemental gallium deposited using trimethylgallium as a gas source and selected deposition conditions during chemical vapor deposition. In an alternative embodiment, the layer 54 may be comprised of a combination of elemental gallium and elemental boron in the form, for example, of a bilayer, such as a gallium layer capped by a boron layer.

With reference to FIG. 4 in which like reference numerals refer to like features in FIG. 3 and at a subsequent fabrication stage of the processing method, the dielectric layer 40 is removed from the horizontal surfaces to expose the portion of the top surface 12 a of the base layer 12 not covered by layer 54, the top surfaces of the collector contacts 22, 24, and the top surface of the emitter 16. A silicide layer 56 is formed on the top surface 12 a of the base layer 12 that includes the dopant-containing layer 54 and may be subsequently used in the process flow to contact the extrinsic base 38.

The silicide layer 56 may be formed by a silicidation process that involves one or more annealing steps to form a silicide phase by reacting a metal layer 55 (FIG. 3) comprised of silicide-forming metal and the semiconductor material contacting the silicide-forming metal. The metal layer 55 may be deposited by, for example, a chemical vapor deposition or physical vapor deposition. The metal layer 55 is located on the dopant-containing layer 54 and adjacent to the semiconductor material of the base layer 12 with which the metal layer 55 is reacted. Candidate refractory metals for the silicide-forming metal include, but are not limited to, titanium (Ti), cobalt (Co), and nickel (Ni). A capping layer comprised of a metal nitride, such as sputter-deposited titanium nitride (TiN), may be applied to cap the metal layer 55.

In a representative silicidation process that involves multiple annealing steps, an initial annealing step may form a metal-rich silicide that consumes the silicide-forming metal and then form silicides of lower metal content that grow by consuming the metal-rich silicides. For example, the initial annealing step may be conducted by heating the silicide-forming metal utilizing rapid thermal annealing (RTA) at an annealing temperature, which may be contingent on the type of refractory metal, of about 400° C. to about 900° C. Following the initial annealing step, any non-converted silicide-forming metal and the optional capping layer may be removed with, for example, one or more wet chemical etches. The silicide layer 56 may then be subjected to an additional annealing step to form a lower-resistance silicide phase. The annealing temperature of the additional annealing step may be greater than the annealing temperature of the initial annealing step.

During the formation of the silicide, the dopant comprising the dopant-containing layer 54 may be thermally driven by diffusion into the sections 34, 36 of the base layer 12 defining the extrinsic base 38 by the annealing steps. The dopant may be activated by a thermal process, such as the thermal process provided by the annealing steps of the silicidation process, to increase the electrical conductivity (i.e., lower the electrical resistivity) of the extrinsic base 38 by increasing the concentration of the dopant of its constituent semiconductor material. The increase in dopant concentration is relative to the original dopant concentration in the base layer 12 and remaining in the intrinsic base 28 after the silicide formation. The diffused length of the dopant into the base layer 12 is greater than the original thickness of the dopant-containing layer 54. Alternatively, a separate annealing process may be used to cause dopant diffusion from the dopant-containing layer 54 into the extrinsic base 38.

The doping of the sections 34, 36 of the base layer 12 may promote low contact resistance with the extrinsic base 38 and low base resistance, as well as increase the gain (i.e., beta) of the bipolar junction transistor 10. The doping of the extrinsic base 38 is accomplished without the use of ion implantation, which could also unwantedly dope a top portion of the emitter 16 with dopant of the opposite conductivity type, and only with the use of one additional mask. The intrinsic base 28 and the extrinsic base 38 are formed from the same base layer 12 formed by epitaxial growth. However, additional dopant added to the extrinsic base 38 through the introduction of the dopant-containing layer 54 into the intermediate device structure is in addition to any dopant provided in-situ during growth.

Sections of the silicide layer 56 are also formed by the silicidation process on the collector contacts 22, 24 and may be subsequently used in the process flow to contact the collector 14. A section of the silicide layer 56 is also formed by the silicidation process on the top surface of the emitter 16 and may be subsequently used in the process flow to contact the emitter 16. Sections of the silicide layer 56 are formed by the silicidation process inside the trenches 18, 20.

With reference to FIG. 5 in which like reference numerals refer to like features in FIG. 4 and at a subsequent fabrication stage of the processing method, middle-of-line (MOL) processing, which includes formation of a dielectric layer 60, contacts 62, 63, 64, and wiring, follows to define a local interconnect structure. Contacts 62 are coupled with the collector contacts 22, 24, contacts 63 are coupled with the extrinsic base 38, and contacts 64 are coupled with the emitter 16. Candidate inorganic dielectric materials for the dielectric layer 60 may include, but are not limited to, borophosphosilicate glass (BPSG), silicon dioxide (SiO₂), silicon nitride (Si₃N₄), fluorine-doped silicon glass (FSG), and combinations of these and other dielectric materials. Dielectric layer 60 may be deposited by any number of techniques including, but not limited to, sputtering, spin-on application, or CVD. Portions of the dielectric layer 60 may at least partially fill the trenches 18, 20 to define trench isolation regions 66, 68 that laterally flank the collector 14.

Back-end-of-line (BEOL) processing follows, which includes formation of additional dielectric layers, via plugs, and wiring for an interconnect structure coupled by the local interconnect structure with the bipolar junction transistor 10, as well as other similar contacts for additional device structures like bipolar junction transistor 10 and CMOS transistors that may be included in other circuitry fabricated on the substrate 11.

The methods as described above are used in the fabrication of integrated circuit chips. The resulting integrated circuit chips can be distributed by the fabricator in raw wafer form (e.g., as a single wafer that has multiple unpackaged chips), as a bare die, or in a packaged form. In the latter case, the chip is mounted in a single chip package (e.g., a plastic carrier, with leads that are affixed to a motherboard or other higher level carrier) or in a multichip package (e.g., a ceramic carrier that has either or both surface interconnections or buried interconnections). In any case, the chip is then integrated with other chips, discrete circuit elements, and/or other signal processing devices as part of either (a) an intermediate product, such as a motherboard, or (b) an end product. The end product can be any product that includes integrated circuit chips, ranging from toys and other low-end applications to advanced computer products having a display, a keyboard or other input device, and a central processor.

A feature may be “connected” or “coupled” to or with another element may be directly connected or coupled to the other element or, instead, one or more intervening elements may be present. A feature may be “directly connected” or “directly coupled” to another element if intervening elements are absent. A feature may be “indirectly connected” or “indirectly coupled” to another element if at least one intervening element is present.

The descriptions of the various embodiments of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiments, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein. 

What is claimed is:
 1. A method of fabricating a device structure, the method comprising: forming a first trench and a second trench in a substrate; forming a collector in the substrate between the first trench and the second trench, wherein the first trench and the second trench establish boundaries for the collector; forming a base layer on a top surface of the substrate over the collector; forming an emitter on a top surface of the base layer; forming a dielectric layer on exposed surfaces of the base layer; forming a hardmask layer on the base layer and the dielectric layer; forming a pattern from the hardmask with a first opening and a second opening; removing portions of a dielectric layer from the top surface of the base layer at positions consistent with the pattern of the first opening and the second opening to form exposed surfaces defined as a first window and a second window in the dielectric layer; limiting deposits of a dopant-containing layer on the exposed surfaces of a first portion on the top surface of the base layer inside of the first window, and a second portion on the top surface of the base layer inside of the second window, wherein the first portion is on a first side of the emitter, the second portion is on a second side of the emitter, and the first window and the second window formed in the dielectric layer bound the first portion and the second portion on the top surface of the base layer; and transferring dopant from the dopant-containing layer into the first portion and the second portion on top of the base layer to define an extrinsic base of the device structure.
 2. The method of claim 1, wherein: the device structure comprises a vertical architecture for a bipolar junction transistor that includes the emitter, the collector, and an intrinsic base defined by a central section of the base layer that is positioned between the emitter and the collector.
 3. The method of claim 2, wherein the deposits of the dopant-containing layer are limited to only areas of the first portion and the second portion on the top surface the base layer where the portions of the dielectric layer were removed.
 4. The method of claim 1, wherein transferring the dopant from the dopant-containing layer into the first portion and the second portion on the top surface of the base layer comprises: siliciding the first portion and the second portion on the top surface of the base layer.
 5. The method of claim 4, wherein the dopant is transferred from the dopant-containing layer into the first portion and the second portion on top of the base layer when the first portion and the second portion on top of the base layer is silicided.
 6. The method of claim 4, wherein siliciding the first portion and the second portion on top of the base layer on which the dopant-containing layer is deposited comprises: depositing a metal layer on the dopant-containing layer; and thermally annealing to react the metal layer with the first portion and the second portion on top of the base layer and to cause the dopant to diffuse into the first portion and the second portion on top of the base layer.
 7. The method of claim 4, further comprising: after the first portion and the second portion on top of the base layer is silicided, forming a contact coupled with the first portion and the second portion on top of the base layer.
 8. The method of claim, 1 wherein transferring the dopant from the dopant-containing layer into the first portion and the second portion on top of the base layer to define the extrinsic base of the device structure comprises: annealing to cause the dopant to diffuse from the dopant-containing layer into the first portion and the second portion on top of the base layer, wherein electrical conductivity of the extrinsic base increases due to diffusion of the dopant.
 9. The method of claim 8, wherein a diffused length of the dopant into the first portion and the second portion on top of the base layer is greater than a thickness of the dopant-containing layer when deposited.
 10. The method of claim 1, wherein the dopant-containing layer comprises boron.
 11. The method of claim 1, wherein the dopant-containing layer comprises gallium.
 12. The method of claim 1, wherein the dopant-containing layer comprises gallium and boron. 